Our source of fat is chiefly animal food and in a smaller measure vegetables; but the fat our food contains is not altogether responsible for the fat in our bodies. Carbohydrates, if in excess of momentary needs, are partly converted into fat and stored as such. A reserve supply of nourishment is thus provided, and is drawn upon only when the food that we consume does not contain as much energy as we expend. WHAT WE DO TO OUR FOOD.--With the exception of water and mineral substances, the food-stuffs must undergo chemical alterations before they are capable of being absorbed into the body; this is the work of digestion. The digestive processes, the main purpose of which is to break up the carbohydrates, proteins, and fats into substances of much simpler chemical structure, begin in the mouth and are not completed until some time after the food has entered the intestine. As the food moves through the alimentary canal, it is mixed with the various digestive juices containing ferments, such as pepsin, which are the active agents of digestion. Although digestive processes go on automatically, they are, in a degree that is far from negligible, influenced by the mind. Thus, cheerfulness promotes digestion, and not infrequently mental depression may be the direct cause of indigestion. Indeed, it is chiefly in regard to the state of the mind of the prospective mother that the existence of pregnancy may be said to have a bearing, whether favorable or unfavorable, upon her digestion. The digestive juices are prepared in glands which lie either within the lining of the alimentary canal or adjacent to it. In the latter event the glands are connected with the canal by means of tubes. These glands must be warned when to pour out their secretion, and their very first warning usually comes from the agreeable sensations experienced when we see, smell, or taste inviting food. If we are hungry, our viands attractive, and our surroundings congenial, the stimulus excites a plentiful secretion of the digestive juices; conversely, the opposite conditions, to some extent, check their flow. The sight of attractive food, as we all know, "makes the mouth water," that is, it calls forth the saliva which contains one of the digestive ferments. Thus, at the beginning of a meal, favorable conditions for digestion are established. The saliva, however, acts only upon starch; and, moreover, its action upon this carbohydrate is weak unless the food is thoroughly chewed and mixed in the mouth. Most of us, perhaps, overlook the importance of mastication, which not only crushes all the food-stuffs, preparing them for efficient digestion, but also stimulates the flow of the digestive juices. Furthermore, by thoroughly masticating our food, we know intuitively when we have had enough, and thus avoid overeating.